Sounds are located in the IWD's so you could extract them, and changing them is entirely possible, though you would have to worry about the differences in the CRC/MD5's of your new file being different from the servers. Not sure how the game would react to new files, but its worth a shot. More than likely VAC isnt going to care as you would get Cannot connect to server errors more than likely.
It's just opening the IWD's with WinRar, finding the spawn themes and replacing the files with proper extensions. It's quite simple just to try, but the game probably doesn't like the change of the file as Heartview said.